Current water distillation approaches typically discard the energy used for evaporation. A simple system consists of an insulating panel that floats on ‘sea’ water, with a black absorber pad on its surface. The wet panel floats underneath a transparent tent. When sea water is heated on the upper surface of the floating panel, it evaporates. The vapor condenses on the inside surface of the transparent tent, as potable water, which is captured at the tent's perimeter. The energy that is used to heat the sea water from ambient to its boiling point, and then to further overcome the latent heat of evaporation, is lost to the atmosphere through the surface of the tent. (Ref: U.S. Pat. No. 7,008,515)
A more efficient simple system consists of a water-heating unit, as above, but the water vapor created is condensed on an array of tubing which carries incoming seawater, and pre-heats it. Transfer of energy occurs from superheated vapor, to raise the temperature of water from ambient to its boiling point but this transfer can't occur to overcome the latent heat of evaporation . . . the transformation of fluid water-to-water vapor which occurs at its boiling point. The reason is that heat transfer will occur only when there is a temperature difference. At the evaporation temperature the energy doesn't move any more. About 14% of the energy used for evaporation may be captured and reused. (Ref: U.S. Pat. No. 4,622,103)
To raise the temperature of one pound of water, one degree F., takes one British Thermal Unit (BTU). So to raise a pound of water from 72 degrees F. (ambient) to 212 degrees F. takes about (212−72=) 140 BTU. To convert one pound of liquid water at its boiling point to one pound of water vapor, takes another 970 BTU, this is the latent heat of evaporation. In the second more efficient simple system above, the heat that is recovered from steam, can be used to ‘preheat’ nearly (970/140=) 7 times as much water up to ‘boiling’ temperature, as was evaporated in the first place. None of the energy recovered by condensing the water vapor can be recovered to overcome the latent heat of evaporation, because eventually the preheated incoming water becomes the same temperature as the vapor. This approach creates a lot of hot seawater, and not much distilled water.
An even more efficient, but more complex system can include a vapor compression pump. Here, steam from the heating system is compressed by the vapor compression pump, so that it gives up its heating energy to ‘preheat’ incoming seawater. ‘Work’ is done by the pump, requiring additional energy to be input, which squeezes the vapor into water. When this occurs on the wall of an array of tubing with ‘incoming seawater’, the energy from the steam compressed into fluid, can be used to overcome the latent heat of evaporation, and steam is created in proportion to the steam that is compressed. (Ref: U.S. Pat. No. 6,508,936)
In summary, an inefficient but simple distillation system doesn't recover the heat energy used to evaporate water. A more efficient but simple system can recover some of the heat energy used to evaporate water, to preheat incoming water (about one seventh of the energy could be effectively recovered). An efficient, but complex distillation system can recover nearly all the heat of evaporation to create more evaporation, but it requires a pump and additional energy to operate the pump (and this operating energy too, might also be recovered to create even more evaporation).
The three scenarios above discuss the thermal efficiency of current distillation approaches. The ability of these approaches to manage miscible fluid combinations interspersed with a simple fluid stream like seawater, is limited.